180 PROCEEDINGS OF 



identical with those of the St. Lawrence river, Sweden and Norway, referred by 

 Mr. Lyell, to the post-pliocene period. 



The upper tertiary is divided in the tablo into three sections, not that 1 would 

 assert positively that there is much, if any difference in age, but because of the 

 peculiar groups and distribution of species. The lowest section is most unlike the 

 two upper, in consequence of exhibiting a group of shells approximating that living 

 in the Gulf of Mexico, but it must bo remembered that some species ranging as far 

 north as Massachusetts, also occur with the fossils, and that probably when these 

 were living in the sea, the peninsula of Florida was yet submerged. This form of 

 tlie coast would have given the Gulf stream more influence in distributing southern 

 forms far northward along the coast. But some change of climate, at the close of 

 this period, seems not improbable from the circumstance that the common estuary 

 fossil of the Gulf of Mexico, Gnathodoii cuncatus, of Gray, was banished from the 

 brackish waters of Georgia, South and North-Carolina, Virginia and Maryland, at 

 this geological epoch. This shell lives so far from the sea, in mud flats, exposed at 

 low tide, that it is not likely to have been destroyed by any change in the Gulf 

 stream, or a receding of its imparted temperature from the coast. If this alteration 

 of temperature can be established by new investigations, the deposits can hardly 

 be of the same age as those of Raritan bay and the St. Lawrence river. In the 

 former locality, there is no evidence of the slightest variation of temperature from 

 the historical period, whilst the latter is supposed to contain a group peculiar to 

 higher latitudes. This, however, wants confirmation, since most of the species are 

 known to exist on the coasts of Maine and Massachusetts. 



Medial Tertiary Period. 



In a recent excursion with my friend Mr. Markoe, I made a hasty examination 

 of some localities in Anne Arundel, Calvert, and St. Mary's counties. Except the 

 lower tertiary at Upper Marlborough, these were chiefly referable to the medial ter- 

 tiary period. The first point of observation was near Fair Haven, Anne Arundel 

 count}', where Mr. Markoe had previously obtained some interesting relics. This is 

 the northern extremity of the formation on the west shore of the Chesapeake ; a lino 

 run from Fair Haven to a point opposite Crane Island, in the Potomac, dividing 

 the lower from the medial tertiary. At Fair Haven we observe a range of coast 

 about fifty feet in elevation. Valleys of denudation sloping to the shore of the 

 Chesapeake, interrupt the continuity of the bank, which presents a front of isolated 

 perpendicular cliffs at irregular intervals. The lowest bod, which is on a level with 

 the tide, is composed of clay, containing a stratum of Ostrea percrassa, a new Pec- 

 ten, P. Humphroysii,* and other undetermined shells. This stratum rises to the 

 height of about five foot above the level of the bay, .A.bovo is a light-colored clay, 

 coutaiiiiug great numbers of black wutcr-worn siliceous casts of small shells, 

 chiefly Turritclla, the species not yet determined. To this succeeds a whitish clay 

 without fossils. 



> Named afici Dr. liumplin js, of St. John's College, at Auiui>ulii. 



